Improved geodetic European very-long-baseline interferometry solution using models of antenna gravitational deformation

Very-long-baseline interferometry (VLBI) is used for establishing global geodetic networks where the coordinates attain a 1-mm level of precision. Technique-dependent bias can degrade the VLBI positioning accuracy if it is present and unaccounted for. Among the potential bias, gravitational flexure of VLBI telescopes can vary the path traveled by the incoming radio signal and induce a bias in the height component of the station position. We process here more than 100 European VLBI sessions spanning 1990-2009 with VLBI time delay/Solve software, as the only VLBI analysis package that can be used to correct signal-path variation (SPV) due to gravitational flexure of VLBI telescopes. Currently, SPV models are neglected in VLBI data analysis. To determine the kinematics of the European area over the last 20 years and to assess the effects of telescope gravitational deformation on geodetic VLBI estimates, we perform two VLBI solutions with and without SPV models for telescopes in Medicina (northern Italy) and Noto (southern Italy). The two solutions differ by 8.8 mm and 7.2 mm in their height components, with this bias being one order of magnitude larger than the formal errors of the estimated heights. SPV models impact uniquely on the height component of stations where SPVs are modeled. Velocities are not affected by the use of the Medicina and Noto SPV models, and we show that the crustal kinematics derived from VLBI does not suffer from a lack of information with regard to the flexure of other telescopes. © 2010 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved

The Italian VLBI Network: First Results and Future Perspectives

A first 24-hour Italian VLBI geodetic experiment, involving the Medicina, Noto, and Matera antennas, shaped as an IVS standard EUROPE, was successfully performed. In 2014, starting from the correlator output, a geodetic database was created and a typical solution of a small network was achieved, here presented. From this promising result we have planned new observations in 2016, involving the three Italian geodetic antennas. This could be the beginning of a possible routine activity, creating a data set that can be combined with GNSS observations to contribute to the National Geodetic Reference Datum. Particular care should be taken in the scheduling of the new experiments in order to optimize the number of usable observations. These observations can be used to study and plan future experiments in which the time and frequency standards can be given by an optical fiber link, thus having a common clock at different VLBI stations

Height bias and scale effect induced by antenna gravitational deformations in geodetic VLBI data analysis

The impact of signal path variations (SPVs) caused by antenna gravitational deformations on geodetic very long baseline interferometry (VLBI) results is evaluated for the first time. Elevation-dependent models of SPV for Medicina and Noto (Italy) telescopes were derived from a combination of terrestrial surveying methods to account for gravitational deformations. After applying these models in geodetic VLBI data analysis, estimates of the antenna reference point positions are shifted upward by 8.9 and 6.7 mm, respectively. The impact on other parameters is negligible. To simulate the impact of antenna gravitational deformations on the entire VLBI network, lacking measurements for other telescopes, we rescaled the SPV models of Medicina and Noto for other antennas according to their size. The effects of the simulations are changes in VLBI heights in the range [-3, 73] mm and a net scale increase of 0.3-0.8 ppb. The height bias is larger than random errors of VLBI position estimates, implying the possibility of significant scale distortions related to antenna gravitational deformations. This demonstrates the need to precisely measure gravitational deformations of other VLBI telescopes, to derive their precise SPV models and to apply them in routine geodetic data analysis

Local effects of redundant terrestrial and GPS-based tie vectors in ITRF-like combinations

Tie vectors (TVs) between co-located space geodetic instruments are essential for combining terrestrial reference frames (TRFs) realised using different techniques. They provide relative positioning between instrumental reference points (RPs) which are part of a global geodetic network such as the international terrestrial reference frame (ITRF). This paper gathers the set of very long baseline interferometry (VLBI)-global positioning system (GPS) local ties performed at the observatory of Medicina (Northern Italy) during the years 2001-2006 and discusses some important aspects related to the usage of co-location ties in the combinations of TRFs. Two measurement approaches of local survey are considered here: a GPS-based approach and a classical approach based on terrestrial observations (i.e. angles, distances and height differences). The behaviour of terrestrial local ties, which routinely join combinations of space geodetic solutions, is compared to that of GPS-based local ties. In particular, we have performed and analysed different combinations of satellite laser ranging (SLR), VLBI and GPS long term solutions in order to (i) evaluate the local effects of the insertion of the series of TVs computed at Medicina, (ii) investigate the consistency of GPS-based TVs with respect to space geodetic solutions, (iii) discuss the effects of an imprecise alignment of TVs from a local to a global reference frame. Results of ITRF-like combinations show that terrestrial TVs originate the smallest residuals in all the three components. In most cases, GPS-based TVs fit space geodetic solutions very well, especially in the horizontal components (N, E). On the contrary, the estimation of the VLBI RP Up component through GPS technique appears to be awkward, since the corresponding post fit residuals are considerably larger. Besides, combination tests including multi-temporal TVs display local effects of residual redistribution, when compared to those solutions where Medicina TVs are added one at a time. Finally, the combination of TRFs turns out to be sensitive to the orientation of the local tie into the global frame

Ground-Based Water Vapor Retrieval in Antarctica: An Assessment

The atmospheric water vapor is an important indicator of the Earth’s climate state and evolution. We therefore aimed at calculating the content and long-term variation of the precipitable water vapor at five coastal Antarctic stations, i.e., Casey, Davis, Mawson, McMurdo, and Mario Zucchelli. To do that, we processed the 12-year time series of GPS and radiosounding (RS) observations acquired at those stations, with the purpose of ensuring the utmost accuracy of the results adopting homogeneous, consistent, and up-to-date processing strategies for both data sets. Using the two fully independent techniques, rather consistent contents and seasonal variations of precipitable water were detected, mainly ranging from 1 (Austral winter) to 10 mm (Austral summer). At each site, correlation coefficients varying from 0.86 to 0.91 were found between the GPS and RS time series, with mean discrepancies ≤0.75 mm. There is no clear indication regarding the possible dry or wet biases of one technique with respect to the other, with only a notable GPS wet bias identified at Mawson and a dry bias at Casey that, nevertheless, correspond to an average difference of < 1 mm on the two series; the biases at the other sites are much smaller. Although extremely small, i.e., ranging from−0.03 to 0.04 mm/year, the linear trends of the series are not always consistent in sign. In accordance with the major climate models, the RS linear trends are mostly positive, whereas depending on the site, GPS exhibits a (very small) decrease or increase in water vapor

Water Vapour Assessment Using GNSS and Radiosondes over Polar Regions and Estimation of Climatological Trends from Long-Term Time Series Analysis

The atmospheric humidity in the Polar Regions is an important factor for the global budget of water vapour, which is a significant indicator of Earth’s climate state and evolution. The Global Navigation Satellite System (GNSS) can make a valuable contribution in the calculation of the amount of Precipitable Water Vapour (PW). The PW values retrieved from Global Positioning System (GPS), hereafter PWGPS, refer to 20-year observations acquired by more than 40 GNSS geodetic stations located in the polar regions. For GNSS stations co-located with radio-sounding stations (RS), which operate Vaisala radiosondes, we estimated the PW from RS observations (PWRS). The PW values from the ERA-Interim global atmospheric reanalysis were used for validation and comparison of the results for all the selected GPS and RS stations. The correlation coefficients between times series are very high: 0.96 for RS and GPS, 0.98 for RS and ERA in the Arctic; 0.89 for RS and GPS, 0.97 for RS and ERA in Antarctica. The Root-Mean-Square of the Error (RMSE) is 0.9 mm on average for both RS vs. GPS and RS vs. ERA in the Arctic, and 0.6 mm for RS vs. GPS and 0.4 mm for RS vs. ERA in Antarctica. After validation, long-term trends, both for Arctic and Antarctic regions, were estimated using Hector scientific software. Positive PWGPS trends dominate at Arctic sites near the borders of the Atlantic Ocean. Sites located at higher latitudes show no significant values (at 1σ level). Negative PWGPS trends were observed in the Arctic region of Greenland and North America. A similar behaviour was found in the Arctic for PWRS trends. The stations in the West Antarctic sector show a general positive PWGPS trend, while the sites on the coastal area of East Antarctica exhibit some significant negative PWGPS trends, but in most cases, no significant PWRS trends were found. The present work confirms that GPS is able to provide reliable estimates of water vapour content in Arctic and Antarctic regions too, where data are sparse and not easy to collect. These preliminary results can give a valid contribution to climate change studies

An overview of the Sardinia Radio Telescope geodetic potential at national and international levels

Sardinia Radio Telescope (SRT) potentials in geodesy are mostly connected to VLBI techniques and they may have a remarkable impact on both national and international geodetic science. A development plan concerning geodetic technical instrumentation should be provided soon, so as to perform geodetic observations with the SRT in the near future. The SRT is being developed within a well-consolidated national environment of geodetic VLBI activities: Medicina and Noto observatories have been continuously participating in geodetic VLBI observations for almost two decades. Matera observatory, whose 20-m VLBI telescope is entirely devoted to geodetic VLBI, is one of the few fundamental geodetic Earth's observatories and is running VLBI experiments since 1990. At a national level, the SRT has the capability to establish a self-consistent Italian VLBI network: it would represent a unique facility in Europe. At an international level, the SRT should be made part of the geodetic International VLBI Network whose operations are supported and coordinated by the IVS (International VLBI Service for Geodesy and Astrometry). IVS promotes research and technological development in geodesy and astrometry, with important outcomes for the astronomical VLBI community, too. In the following sections, the contribution of the SRT to space geodesy, geophysics and related research fields will be outlined